The goal of this proposal is to characterize the agrin/MuSK signaling pathway. Both agrin and MuSK are required for the formation of differ- entiated neuromuscular junctions. However, to date, it is unclear as to what is downstream of MuSK in this pathway. The specific aims of this proposal are: to identify molecules that bind to the intracellular domain of MuSK; to characterize the role of these components in acetylcholine receptor clustering (AChR); and to determine the role of tropomyosin in AChR cluster and neuromuscular junction formation. A yeast two-hybrid screen will be utilized in order to identify potential MuSK binding polypeptides. Subsequently, these binding proteins will be characterized using immunofluorescence, immunoprecipitations, deletion and co- transfection analysis to determine their role in AChR clustering. Results from these experiments will further the understanding of the development of neuromuscular junctions and neurodegenerative diseases including muscular dystrophies and myasthenia gravis.